The present invention is related to semiconductor based image sensing devices and, more particularly, to a method and apparatus for use with column output circuits for an X-Y addressable pixel array.
There are numerous prior art teachings relating to X-Y addressable imagers, which are semiconductor based imagers that can be either passive pixel sensor (PPS) devices or active pixel sensor (APS) devices. The difference being that PPS devices do not provide active circuit elements (such as amplifiers) allocated on a pixel-by-pixel basis. Instead PPS devices generally output directly from the pixel onto a column bus. The APS devices, on the other hand, typically provide amplifiers on a pixel-by-pixel basis. CMOS active pixel image sensors have been recently commercialized to supplement and replace charge-coupled devices (CCD) in many applications. CMOS image sensors have advantages in low-power operation and being able to be integrated with on-chip signal processing and control functions making them suitable for systems requiring low-cost portable imaging devices.
Very often, multiple column circuits are used to process pixel signals in parallel to achieve the low power operation. In PPS devices, column circuits receive pixel data directly and hold it in a multiplexed manner until analog-to-digital conversion (ADC) can be performed. In APS devices not only will the column circuits perform the function of a simple multiplexor, but correlated double sampling (CDS) is additionally accomplished prior to analog to digital conversion. Typically, the column output circuits that are provided within the same chip as the PPS and APS image sensor devices to perform these functions, have been limited to rows designed for the injection of charges of a predetermined voltage level. These analog-injection or charges are used to identify the characteristics of the column output circuits. There is a shortcoming in these on chip injection circuits as employed by prior art PPS and APS devices in that they do not provide for an analysis of the spatially varying frequency response or the time frequency response characteristics of the column output circuits.
Form the foregoing discussion, it should be readily apparent that there remains a need within the art for a method and apparatus that provides for an analysis of the characteristics of the column output circuits as a response to a spatially varying frequency stimuli. At a fixed timing clock frequency, the response to spatial variations is then directed translatable into time frequency response.
This present invention addresses the aforementioned needs within the art by disclosing a method and apparatus that provides for an analysis of the characteristics of the column output circuits as a response to a spatially varying frequency stimuli. The preferred embodiment illustrates that alternating bright and dark pixels wherein the periods of bright and dark are varied by changing the consecutive number of pixels that are bright and dark. Accordingly, the present invention provides a technique and architectural design to achieve efficient testing of the multiple parallel output column circuits integrated with an X-Y addressable image sensor by using on-chip analog injection signals. This invention is particularly useful in testing high-speed output column CDS circuits for a high frame rate image sensor fabricated using a Complementary Metal Oxide Semiconductor (CMOS) active pixel sensor (APS) technology incorporating a pinned photo-diode. The technique of the present invention allows for testing the frequency responses of the output column circuits by observing of the output signals either in analog form, or digital bits if an on-chip ADC is used, by measuring only the output response to the injection signals without requiring the application of any external time varying input signals. By employing a fixed timing clock frequency, the response to spatial variations is then directed translatable into time frequency response.
These features are provided within the present invention by an X-Y addressable image sensor with built-in, self-testing provisions of column output circuits comprising: a plurality of pixels arranged in a plurality of rows and a plurality of columns to form the X-Y addressable image sensor; a plurality of column output circuits; at least one row used as an injection row, the injection row being electrically coupled to a predetermined signal level; selection means for enabling the pixels in the injection row to output to the column output circuits; and means for altering the predetermined signal level applied to the pixels of the injection row.